1. Field of the Invention
This invention relates to the field of oxygen gas detection, and more particularly relates to electrolyte-formulations and methods of using those formulations to inhibit undesired deposit formation on cathodes used in the detection equipment. More specifically, the electrolytes and method suppress the dissolution of silver chloride from silver/silver chloride reference electrodes and the attendant deposition of silver on neighboring electrodes.
2. Description of the Related Art
Polarographic sensors of the type disclosed in Clark U.S. Pat. No. 2,913,386 have become increasingly popular in medical, biological, industrial and environmental applications for detecting and measuring gases such as oxygen dissolved in a liquid medium. A Clark-type sensor includes a sensor electrode and a reference electrode exposed to an aqueous electrolyte solution which is isolated from the liquid medium by a semi-permeable membrane. In the specific case of a dissolved oxygen sensor, oxygen diffuses from the liquid medium through the semi-permeable membrane into the electrolyte solution at a rate proportional to the oxygen partial pressure in the liquid medium. The diffused oxygen migrates toward the sensor electrode (which is configured as a cathode) where it is reduced in accordance with the following reaction: EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (I)
This reaction induces an electron flow from the sensor electrode having a current magnitude proportional to the oxygen partial pressure in the liquid medium.
Silver/silver chloride electrodes are commonly used as a reference electrode in such Clark sensors due to the stability of its oxidation potential over a wide temperature range in a chloride-based electrolyte solution. The oxidation potential of a silver/silver chloride electrode is established by the reaction: EQU Ag+Cl.sup..multidot. .fwdarw.AgCl+e.sup.-. (II)
Chloride-based electrolyte solutions perform two functions in a polarographic sensor having a silver/silver chloride electrode. They supply ions in the electrolyte solution to conduct the current necessary to maintain the reactions at the sensor and reference electrodes, and they supply the Cl ions to this anode/reference electrode for the oxidation of Ag in accordance with the preceeding oxidation of Ag (II). Over time, the reaction near the reference electrodes exhausts the chloride ion concentration, so that the electrolyte solution must be replaced periodically to maintain the accuracy of the sensor.
One drawback to the use of silver/silver chloride reference electrodes in a chloride-based electrolyte solution is the plating or formation of Ag onto the sensor or cathode electrode. The solubility of silver chloride in an aqueous solution increases with increasing chloride ion concentration in the solution. Silver chloride is practically insoluble in pure water, having a solubility product constant of approximately 1.times.10.sup.-10. On the other hand, AgCl readily dissolves into a high chloride medium to form Ag(Cl.sub.2).sup.-. The total solubility of silver chloride in the forms of Ag.sup.+, AgCl and Ag(Cl.sub.2).sup..multidot. is plotted as a function of the logarithm of chloride ion concentration in FIG. 1. As shown by the curve 10 in FIG. 1, the total solubility of silver chloride increases rapidly above a chloride ion concentration of approximately 1.times.10.sup.-2 M.
Thus, a common problem with Clark-type sensors using silver/silver chloride reference electrodes and chloride-based electrolyte solutions is the dissolution of silver chloride from the reference electrode and the deposition of silver in the form of dendrites on the sensor electrode. Dendrite formation at the cathode of an oxygen sensor has resulted in the following two symptoms: (1) the oxygen reading gradually rises with time at a rate of about 0.5-1% per hour and (2) the oxygen reading fluctuates frequently and drastically. These two symptoms can be explained by the deposition of silver in the following way: the addition of silver to the cathode increases the area of the cathode which in turn increases the sensor current. Additionally, when dendrites grow large enough in size, some of them break off from the cathode which causes the reading to drop. Then, these broken off dendrites sometimes attach themselves back to the dendrites at the cathode, causing a sudden rise in the oxygen reading.
Various techniques have been proposed to suppress the deposition of silver on the sensor electrodes of Clark-type sensors. For example, Molloy U.S. Pat. No. 3,406,109 proposed placing a moat or groove between the sensor electrode and the reference electrode to interrupt the migration of silver ions toward the sensor electrode. While the placement of a moat between the electrodes as shown in Molloy does reduce the level of silver deposition in certain sensor geometries, the moat does not completely prevent such deposition. Furthermore, space limitations preclude the placement of such moats in a large number of commercially available sensor configurations.
Clark U.S. Pat. No. 3,380,905 and Alena et al. U.S. Pat. No. 4,803,991 each proposed using a quinhydrone reference electrode rather than a silver/silver chloride electrode by adding a small amount of hydroquinone to the electrolyte solution. While such proposals are effective in inhibiting silver deposition on the sensor electrode, they complicate the structure of the sensor and would require that existing sensors using silver/silver chloride reference electrodes be redesigned to accommodate different current levels. Thus, there remains a need for a solution to the problem of silver deposition on the cathodes of Clark-type sensors which is compatible with existing sensor configurations using silver/silver chloride reference electrodes.